The diagram shows a typical human tooth. All human adult teeth conform to the same basic structure shown here.

There is a visible crown projecting above the gum (medically known as the gingiva). The root of the tooth is embedded in the alveolar bone of the jaw and is covered in a layer of cementum.
Children have 20 deciduous teeth and adults have 32 permanent teeth.

In the adult the teeth are specialized to form different functions during mastication (or chewing). The molars and pre-molars grind food. The incisors cut food and the canines tear at food.

Enamel is an extremely hard material, which covers the crown and the root of the teeth. This protects the more delicate inner structures of the tooth and provides the hard surface required for the functions outlined above. The inner layer of the tooth is formed from dentine, which has a similar structure to bone.

In the center of the tooth there is a pulp cavity, which contains nerves and blood vessels. It is the stimulation of these nerves, which causes the intense pain associated with dental caries.
Dental caries (tooth decay) is one of the most common of all disorders, second only to the common cold. It usually occurs in children and young adults but can affect any person. It is the most important cause of tooth loss in younger people.

Bacteria are normally present in the mouth. The bacteria convert all foods-especially sugar and starch-into acids. Bacteria, acid, food debris, and saliva combine in the mouth to form a sticky substance called plaque that adheres to the teeth.

It is most prominent on the grooved chewing surfaces of back molars, just above the gum line on all teeth, and at the edges of fillings. Plaque that is not removed from the teeth mineralizes into calculus (tartar). Plaque and calculus irritate the gums, resulting in gingivitis. It is well known that tooth decay can lead to the destruction and eventual loss of teeth.

The acids in plaque dissolve the enamel surface of the tooth and create holes in the tooth (cavities).
   
Cavities are usually painless until they grow very large inside the internal structures of the tooth (the dentine and the pulp at the core) and can cause death of the nerve and blood vessels in the tooth, resulting in tooth abscess.


1.

Early stages : Acid dissolve the enamel in the crown of the tooth

2.
Moderate tooth decay : Here acids attack the dentine and bacteria invade the cavity.
3.
Advanced tooth decay : Inflammation of the pulp.
4.
Necrosis (death) of the pulp tissue.
5.
Periapical abcess forms at the apex of the root.

Acids begin to dissolve tooth enamel within 20 minutes after eating, the time when most bacterial activity occurs. Untreated tooth decay can result in death of the internal structures of the tooth with eventual loss of the tooth.

Dietary sugars and starches (carbohydrates) increase the risk of tooth decay. The type of carbohydrate and the timing of ingestion are more important that the amount. Sticky foods are more harmful than non-sticky foods because they remain on the surface of the teeth. Frequent snacking increases the time that acids are in contact with the surface of the tooth.

Oral hygiene is the primary prevention against dental caries. This consists of personal care (proper brushing at least twice a day and flossing at least daily) and professional care (regular dental examination and cleaning, at least once a year).
Chewy, sticky foods (such as dried fruit or toffee) are best if eaten as part of a meal rather than as a snack. If possible, brush the teeth or rinse the mouth with water after eating these foods. Minimize snacking, which creates a constant supply of acid in the mouth. Avoid constant sipping of sugary drinks or frequent sucking on sweets or mints unless sugar free.

The use of dental sealants is a good means of cavity prevention. Sealants are thin plastic-like coating applied to the chewing surfaces of the molars. This coating prevents the accumulation of plaque on these vulnerable surfaces. Sealants are usually applied on the teeth of children, shortly after the molars erupt. Older people may also benefit from the use of tooth sealants.

Fluoride is often recommended to protect against dental caries. It has been demonstrated that people who ingest fluoride in their drinking water or by fluoride supplements have fewer dental caries. Fluoride that is ingested when the teeth are developing is incorporated into the structure of the enamel and protects it against the action of acids.

Topical Fluoride is also recommended to protect the surface of the teeth. This may include a fluoride toothpaste or mouthwash. Many dentists include application of topical (applied to a localized area of the teeth) fluoride solutions as part of routine visits.